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Pinus edulis
is sometimes described as occurring in the New York Mountains of extreme eastern California, but other authors assign these trees to
Pinus monophylla
. These trees have also been described as a species,
Pinus californiarum
D.K. Bailey, which has also been described as a subspecies (
P. monophylla
subsp.
californiarum
(D.K. Bailey) Zavarin) or variety (
P. monophylla
var.
californiarum
(D.K. Bailey) Silba) of
P. monophylla
.

The piñon-juniper woodland is one of the principal forest types of western North America. Woodlands of this type, dominated by
Pinus edulis
, cover approximately 14.9 million ha in Arizona, Colorado, New Mexico, and Utah (Ronco 1990). Most such woodlands are dominated by one or two species of pine and another one or two species of juniper. The principal pines involved are
Pinus edulis
,
P. monophylla
, and
P. cembroides
, and the principal junipers are
Juniperus occidentalis
,
J. monosperma
,
J. osteosperma
,
J. deppeana
and
J. flaccida
. However, a good variety of other, sometimes rather unusual combinations may occur. For instance, at the northern extreme, the Missouri Breaks of eastern Montana have
Pinus flexilis
-
Juniperus scopulorum
woodlands, whereas Mexico has stands that contain some very rare species of both pine and juniper. Regardless of its dominant tree species composition, the piñon-juniper woodland is of enormous ecological importance because the dominant trees create a structure that produces habitat diversity, attenuates soil erosion and microclimatic extremes, retains snow cover and enhances soil moisture, supports very high diversity of both cryptogamic and vascular vegetation, and provides an important food source (pine nuts and juniper "berries") for many species of birds, mammals, and insects. These resources are in turn available to humans, who historically have exploited them primarily for grazing domestic animals, and in this connection the piñon-juniper woodland is of great economic importance.

Although Americans tend to see piñon-juniper as a hot desert vegetation type,
Pinus edulis
in particular occupies a relatively cold, relatively mesic niche within the piñon-juniper type. Annual precipitation varies with geography and elevation from about 250-560(-690) mm, and varies from summer-wet to winter-wet, though some summer rainfall occurs throughout the species' range (Ronco 1990).
Pinus edulis
tends to give way to junipers or desert shrubs on dry sites, and to forest trees such as
Pinus ponderosa
on wet sites. Temperatures in the species' range vary from (-35-)4-16(-44)°C, with January means of -10-6°C and July means of 20-27°C (Ronco 1990). Hardy to Zone 5 (cold hardiness limit between -28.8°C and -23.3°C) (
Bannister and Neuner 2001
).

One of the most ecologically interesting things about piñon pines is their relationship with the birds that gather, cache and eat their seeds, and upon which the trees are completely dependent for seed dissemination. This relationship is wonderfully detailed in two books by Ron Lanner (Lanner
1981
,
1996
). Because the seeds are large and wingless, they cannot be disseminated by wind. Instead, the seeds are gathered by four species of corvids: the Clark's nutcracker (
Nucifraga columbiana
), Steller's jay (
Cyanocitta stelleri
), Mexican jay (
Aphelocoma ultramarina
), and pinyon jay (
Gymnorhinus cyanocephalus
). Each of these will gather and cache huge numbers of seeds for consumption during the winter months. Some seeds are not recovered by the jays, and germinate to produce new seedlings. Steller's and Mexican jays collect seed only from open cones, but pinyon jays and Clark's nutcrackers forage from green cones, and then from open cones as the season progresses. Clark's nutcrackers and Steller's jays probably do not effectively disseminate the seeds because their caches are located in ponderosa pine and mixed conifer forests or in the ecotone above pinyon-juniper woodlands. Mexican jays and pinyon jays, however, cache seeds in woodland areas, the former in small, local territories, whereas the latter transport seeds up to 12 kilometers. Thus, the pinyon jay is the most important of the four species because it forages from both green and mature cones, disseminates the seeds across significant distances, and sites caches in locations where successful regeneration may occur. Pinyon jays can carry an average of up to 56 seeds in an expandable esophagus, and most seeds are cached individually rather than in groups, so there is a good dispersal of the potential seedlings. Pinyon jays live in flocks of 50 to 500 birds, and it has been estimated that during a substantial seed year in New Mexico, about 4.5 million seeds were cached by a single flock (Ronco 1990).

Big tree

Oldest

A crossdated age of 973 years from sample SUNB2522, collected in NE Utah by Schulman in 1956 (
Brown 1996
). Trees more than 750 years old are reasonably common, given that you can find a relatively fireproof site where they have not been killed by ranchers who, especially from the 1950s to the 1990s, would cheerfully bulldoze a thousand-year-old tree in the hope that a little grass would grow up in its stead.

Dendrochronology

Pinus edulis
has been very widely used since the development of dendrochronology; a recent search found it referenced in 147 papers. In the early 20th Century crossdating techniques were used to assign calendar dates to timbers used in the construction of early Native American cliff dwellings and other structures. Later the species was used to develop climate reconstructions, first at the local scale and then as part of large tree-ring data networks covering sub-continental scales. In recent years the uses of the species have further expanded to include streamflow reconstructions, studies of erosion and alluviation, forest demography and succession, forest growth responses to climate change and changing carbon dioxide concentrations, and a host of more esoteric problems in archeology, tree physiology, climate studies, ecology, and geology. More than almost any other tree species, it acts as an articulate witness of all that it experiences in its long lifetime.

Ethnobotany

The seeds are much eaten and traded by Native Americans (
Kral 1993
), and by others who are lucky enough to partake of the harvest. The wood was formerly used in construction by Native Americans, and is still often used for fenceposts and firewood. Due to their ecological importance, management of
Pinus edulis
woodlands is a major concern throughout their area of distribution. Principal management themes include wildfire control, grazing, invasion by weedy forbs and shrubs, and "grassland invasion" due to fire suppression. The literature on this subject is immense; search for "pinyon juniper management" to see some examples.